To determine if tissue infection with MAC occurs prior to sustained bacteremia by obtaining bone marrow biopsies and blood for quantitative cultures from subjects at high risk for DMAC. To examine the relationship between immune dysregulation and MAC disease by identifying immune profiles correlated with risk for MAC disease. To determine whether the abnormalities in immune cell populations and functions found in MAC-infected subjects and in asymptomatic subjects with low CD4+ counts can be reversed by HAART. To evaluate the relationship between changes in HIV RNA levels after HAART and changes in relevant immune parameters. To evaluate the relationships between baseline plasma HIV-1 RNA levels, relevant immune function abnormalities, and MAC infection status. To evaluate the diagnostic utility of rapid and potentially more sensitive new MAC detection methods by correlation with quantitative cultures of blood and bone marrow. To examine the incidence of intestinal and respiratory colonization of subjects with DMAC and compare the strain identities and virulence properties of colonizing strain(s) with MAC isolate(s) obtained from blood and bone marrow.